The present invention relates to a magnetic shield for use in a color CRT (Cathode Ray Tube) and, more particularly, to a method of forming a blackening film on the surface of a magnetic shield efficiently.
It is a common practice with a color CRT for television or display, particularly a color CRT including a shadow mask, to position a mask frame at the inside of a face panel and cause it to support the shadow mask. A magnetic shield is affixed to the mask frame by clamps or similar assembling members in order to protect an electron beam issuing from an electron gun from the influence of, e.g., terrestrial magnetism. Usually, a black oxide film (Fe.sub.3 O.sub.4), or blackening film as referred to hereinafter, is formed on the surface of the magnetic shield in the same manner as blackening films formed on the mask frame and shadow mask. The blackening film of the magnetic shield serves to absorb or diffuse light and electron beam, to radiate heat , and to prevent metallic surfaces from forming rust during heat treatment. Some different blackening methods for forming such a blackening film have been proposed in the past. However, the conventional methods are low in working efficiency and in quality.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication No. 62-126524.